As shown in FIGS. 12 and 14, generally, an impeller 101 used for rotary machines, such as a centrifugal compressor, has a fixed hub portion 112 of a rotating shaft S, a disc-shaped disc 107 that is provided integrally with the hub portion 112, a shroud 108 that is arranged so as to be spaced apart from the disc 107 in the axial direction of the central axis L, and a plurality of blades 106 that are provided in a circumferential direction and connect the disc 107 to the shroud 108. In this type of impeller, a portion surrounded by the side surfaces of the blades 106 and two mutually facing surfaces of the disc 107 and the shroud 108 formed a flow passage 103 for compressing air. Additionally, by shrink-fitting the hub portion 112 to the rotating shaft S of the rotary machine, the impeller 101 is fixed to the rotating shaft S.
The flow passage 103 opens toward a first end side of the central axis L on the inner peripheral side, curves gradually so as to be directed to a radial outer peripheral side, and opens toward the radial direction on outer peripheral side. That is, the flow passage 103 is formed in a curved shape as viewed from the circumferential direction in order to direct a fluid, which is introduced from the first end side along a second end side, to the radial outer peripheral side (particularly, refer to FIG. 14). Moreover, as shown in FIG. 13, as the blades 106 are obliquely connected to the disc 107, the compression performance of the impeller 101 is improved, and thereby, the flow passage 103 assumes a complicated three-dimensional shape.
As a method for manufacturing the impeller 101, a method in which the blades 106 and one of the disc 107 and the shroud 108 are integrally formed, the other of the disc and shroud is separately manufactured, and these disc and shroud are integrated by welding or brazing, is known. Additionally, since the impeller requires high rigidity, a one-piece impeller with high strength reliability is manufactured by shaving out the disc 107, the shroud 108, and the blade 106 from a single base material (for example, refer to PTL 1).